It is known that the refractive index of germanium-doped, SiO.sub.2 -based fibres (among others), can be changed by exposing the fibre to ultraviolet radiation within certain absorption intervals. The ultraviolet wavelengths used to create fraction index changes in holographic page-writing methods lie mainly within germanium-related absorption bands with a maximum at approximately 195 nm and approximately 240 nm, although other wavelength intervals have also been used, these latter wavelength intervals normally requiring much longer exposure times, however. It is possible to produce with holographic page-writing methods periodic refractive-index variations, so-called Bragg gratings, that function as wavelength selective mirrors or filters, with several applications within, e.g., telecommunications and laser or sensor applications.
Fibre gratings are described in the document "Fibre Gratings", Physics World, October 1993, Philip ST. J. Russell et al, pp. 41-46, and also in PCT publication WO 94/00784.
Although the actual process lying behind these index changes has not been fully established, it is generally considered that germanium defects--the concentration of Ge.sup.2+ (c.f., for instance, U.S. Pat. No. 5,157,747, Atkins et al) is the main reason for the resultant photosensitivity. The photosensitivity of a material is, e.g., its ability to change its refractive index upon given exposure to electromagnetic radiation. Although the photosensitivity of fibre can be enhanced in many different ways, the method used is still highly dependent on the use of wavelengths of approximately 195 nm and approximately 240 nm. Sensitivity to ultraviolet light can be enhanced by doping with more GeO or GeO.sub.2 and/or B.sub.2 O.sub.3.
U.S. Pat. No. 5,500,031, Atkins et al, teaches a method of increasing the refractive index of glassy material, by applying heat in conjunction with hydrogen sensitization. Such increases in refractive index are not temperature-stable at temperatures above 600.degree. C. This patent specification teaches solely a method that is aimed at causing chemical reactions to take place over the space of time of some seconds and for temperatures higher than 500.degree. C., and not to cause diffusion of material that has diffused into the material or of doping substances in the material. In order to cause diffusion, the material is heated to temperatures of from 800 to 1100.degree. C. and over much longer times, for instance over minutes or hours.
It has been possible to increase the photosensitivity of certain fibres or waveguides, by diffusing hydrogen thereinto.